The invention relates to switched reluctance electric motors, and more particularly to position and speed detection without a shaft position sensor.
A switched reluctance motor includes a stator having a plurality of circumferentially spaced stator poles having windings therearound, and a rotor rotational about the motor shaft axis and having a central annular hub with a plurality of circumferentially spaced rotor poles extending radially outwardly from the hub. Each stator pole has an inner pole face subtending an arc along a circumference. Each rotor pole has an outer pole face subtending an arc along a second circumference concentric with and spaced slightly radially inwardly of the first circumference by a small clearance gap.
The stator windings are energized in a cyclic sequence to create a rotating magnetic field producing torque on the rotor which rotates to follow the rotating magnetic field. Generally, when the rotor reaches a maximum alignment position relative to the presently energized stator phase winding, the next phase should then be energized, to maximize torque production. The change from one motor state energizing one phase winding to the next motor state energizing the next phase winding is called commutation. It is desirable to know the position of the rotor, in order to select the proper commutation timing and phase energization, and hence provide the optimum magnetic field pattern for producing optimum torque on the rotor.
It is known in the prior art to sense rotor position with a shaft position sensor separate from the motor control, such as a Hall effect device, to provide properly timed switching commutation signals to the motor control. A drawback of shaft position sensors is that they add cost to the motor control, and require additional wiring. The additional wiring requirement is particularly objectionable when the motor is used to drive an air conditioning or refrigerator compressor which is hermetically sealed in a container, because the additional wires must be brought out of the hermetically sealed container, which further adds to the cost.
Various sensorless motor control commutation systems are known in the prior art. These systems monitor a changing motor parameter such as voltage, e.g. back EMF, current, e.g. regeneration current, or reluctance, e.g. by pulsing a phase winding with a test current. A drawback of these systems is that they require complex electronic circuitry for monitoring and discriminating the sensed waveforms. A further drawback is that the resultant waveform signal is not constant, but varies with motor speed and torque.
The present invention provides pole structure creating a change in motor inductance at a given rotor position. One or more irregularities, e.g. notches, are provided in one or more stator pole faces and/or one or more rotor pole faces, to create a unique pattern in the otherwise standard inductance profile of the motor, to allow rotor position detection. The irregularity causes a perturbation or blip in the inductance profile of the motor during rotation of the rotor, indicative of the position of the rotor relative to the stator. The frequency of perturbations indicates motor speed.